1. Field of the Invention
The invention concerns a method for the fabrication of a scintillator, more particularly designed for the input screen of an X-ray image intensifier tube.
It also concerns a scintillator obtained by the application of a method such as this.
2. Description of the Prior Art
X-ray image intensifier tubes are well-known in the prior art. They are used to convert an X-ray image, representing the absorption of X-rays by the structure to be depicted, into a visible image. Devices such as this are widely used for medical observation. An image intensifier tube is formed by an input screen, an electron optical system and an observation screen. The input screen has a scintillator which converts the X-rays into visible photons. These visible photons then strike a photocathode, generally formed by an alkaline antimonide. This antimonide, thus excited, generates a flow of electrons. This flow is then transmitted by the electron optical system which focuses the electrons and directs them to an observation screen, formed by a luminograph, which, then emits a visible light reconstituting the X-ray image. This light can then be processed, for example, by a television, cinema or photographic system.
The scintillator of the input screen is generally formed by cesium iodide deposited by vacuum evaporation on a substrate. The substrate is generally formed by a aluminium cap with a spherical or hyperbolic profile. The thickness of the cesium iodide deposited generally ranges from 150 to 500 microns. The cesium iodide is deposited in the form of needles with a diameter of 5 to 10 microns. Since the refractive index of cesium iodide is 1.8, the advantage of a certain fiber optic effect is obtained. This effect minimizes the lateral diffusion of light within the scintillating material. A scintillator of this type is, for example, described in the French patent application No. 85.12.688 dated 23rd Aug. 1985.
The resolution of the tube depends on the capacity of the cesium iodide needles to properly channel the light. It is therefore useful to reduce their diameter. It also depends on the thickness of the cesium iodide layer. An increase in this thickness harms the resolution. On the contrary, the greater the thickness of cesium iodide, the more X-rays are absorbed. Hence, a compromise has to be found between the absorption of X-rays and resolution. To this effect, the invention proposes an improvement enabling a reduction in the mean diameter of the cesium iodide needles.